PORT TOWNSEND — The level of a potentially lethal nerve toxin in Anderson Lake is almost 300 times above the safety threshold.

The lake in Anderson Lake State Park west of Chimacum, remains closed to all recreational use, as it has been for most of the summer because of toxins created by blue-green algae.

The park around the lake remains open for use.

“We just want to stress to the public that this is a dangerous level of anatoxin-a in Anderson Lake,” said Michael Dawson, water quality lead with the Jefferson County Public Health Department.

“Anatoxin is of particular concern because it’s a neurotoxin,” he said.

It can cause paralysis and stop respiration.

“It affects animals and humans very quickly,” within minutes or hours of exposure, Dawson said.

“It can be lethal.”

The first indication of poison in the popular trout-fishing lake was six years ago, when two dogs died after drinking lake water on Memorial Day weekend. Both were thought to have been killed by anatoxin-a.

Said Dawson on Friday: “We are stressing that people avoid all kinds of contact in Anderson Lake right now, both for themselves and their pets.”

The latest test results, received Friday, found a level of 298 micrograms of anatoxin-a per liter of water in the lake.

The safety threshold is 1 microgram per liter.

“This is a high level of toxin for this time of year,” Dawson said, adding that “the public may be less wary in the fall, but these are still dangerous conditions.”

A heavy bloom of algae with scum extends over the entire lake — also unusual for the autumn months.

Said Greg Thomason, who took the sample that was tested last week: “I’ve never seen the algae so thick in five years of doing this [testing].

The level of anatoxin-a began climbing in the lake in September, after two weeks of tests that found so little anatoxin-a in the lake that the county health department had recommended it be reopened.

State Parks did not reopen the lake, and after the two weeks of low readings, the level of toxin began climbing again.

Researchers don’t know why the toxins are coming up now — but it isn’t because of a new type of blue-green algae, Thomason said.

Thomason had reported late last month that the King County Environmental Lab had found a type of algae never seen before in Anderson Lake, or any East Jefferson County lake, and that it could be responsible for the almost unseasonable toxin level rise.

Last week, he said that wasn’t true.

The lab had called woronichinia, a type of blue-green algae known to live in Anderson Lake, by an old name, coelosphaerium, Thomason said.

“It was a mistake on the part of the lab,” he said.

Since woronichinia has been in the lake all along — and therefore isn’t likely to be responsible for an usual rise in toxins — researchers are left without an idea of why anatoxin-a is skyrocketing now.

“We really don’t know anything more about the cause of this bloom,” Dawson said.

“But certainly weather conditions seem to have been favorable for algae growth.”

Researchers know that warm weather fuels algae growth when sufficient nutrients, such as phosphorus, are present.

Anderson Lake, once the site of a dairy farm that closed in the middle of the last century, may contain high levels of phosphorus, which is present in livestock waste.

Researchers do not know why some species of blue-green algae will suddenly begin to produce toxins.

Microcystin, another toxin created by blue-green algae, was measured at 5.4 micrograms per liter in Anderson Lake.

That’s just below the safety threshold of 6 micrograms per liter for microcystin, which can cause skin irritation and nausea over the short term and liver damage if ingested over a long period of time.

Other lakes in East Jefferson County are safe to use for recreation.

Gibbs Lake and Lake Leland, both of which have moderate blooms with patchy scum, contained traces of anatoxin-a.

A trace of microcystin was found in Gibbs Lake, south of Port Townsend, while none was detected in Lake Leland, north of Quilcene.

No sample was taken from Crocker lake, which is near the U.S. Highway 101-state Highway 104 junction.

Caution signs remain posted at Leland, Gibbs and Crocker lakes because they contain the type of algae known to create poisons at times.

No toxic blue-green algae has been reported in Clallam County, where health officers do not test for toxins; instead, they visually monitor lakes for signs of algae bloom.